Most new turners have a modest sized shop, so something in the “Midi” range might be a good place to start. You can still turn reasonably sized bowls, plates and hollow forms. As you become more proficient you may want to scale up ( you don't have to) your lathe; use the same evaluation process as you did to choose your first lathe. Keep in mind what you will eventually use a larger lathe for. Do you really need a 20” swing when you are more than likely to turn 14”/15” platters, or mid sized salad bowls and a 16” swing would suffice. Consider the bed length, are you going to do architectural turning (need a long bed), if you are going to do mid sized railing spindles or spindle turning the bed length is not as critical. I own both a midi and a full sized lathe and use the one that suits the job at hand. The midi typically for small items such as jewelry, miniatures, small bowls, pens, etc. The larger lathe is for very large bowls, architectural work, platters and large hollow forms. The following are some very general criteria to help you make that buying decision.

What can you afford? Look for the best product in that price range. 

Read the reviews 5star to 1star. What do those reviews say about the product and how it is being used. Even 1 star reviews give you a clue as to either the quality of the product or point out that the user bought the product for a purpose without considering how it would perform or if it is the right product for what they intend to do.

When you have a short list, find a store where you can do a hand on inspection of the product. How does it look and feel. If you can't find a close by store talk to someone who owns the brand you are interested in. Most club members have a wide variety of lathes, you should be able to find a member to talk to.

Other considerations.

What is the lathe speed range. How easy is it to change the speed (variable or belt adjustment). What is the slowest speed; can it go low enough (100rpm/200rpm) to put an unbalanced piece and not have the lathe walk across the shop.

Used is not necessarily bad. Is someone leaving woodturning and selling their equipment. This may be an opportunity to get a new lathe at or below retail/wholesale pricing. Just make sure you know the source and inspect the product before you buy.

A simple test, do the drive center and tail-stock center align ---> <---. If not, can the head-stock be adjusted to have the centers align. You do not want to do spindle turning that looks like it was offset turned.

What is the motor rating. If it is under-powered you will not be able to get the job done efficiently. For a midi' I would suggest a 3/4hp motor as a minimum.

For a larger lathe, not only do you want to look at horsepower (1.5hp or larger), you also want to look at the operating voltage. Is it 110v or 220v; this can impact overall lathe performance.